In this chapter, you will learn that

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9

Muscles and Muscle Tissue

WHY THIS

MATTERS

In this chapter, you will learn that

Muscles use actin and myosin molecules to convert the energy of ATP into force

beginning with

next exploring

9.1 Overview of muscle types, special characteristics, and functions

then exploring

Skeletal muscle

and investigating

then asking

Smooth muscle

and asking

9.2 Gross and microscopic anatomy

and

9.3 Intracellular structures and sliding filament model

9.4 How does a nerve impulse cause a muscle fiber to contract?

and

9.5 What are the properties of whole muscle contraction?

and

9.6 How do muscles generate ATP?

and

9.7 What determines the force, velocity, and duration of contraction?

and

9.8 How does skeletal muscle respond to exercise?

9.9 How does smooth muscle differ from skeletal muscle?

and finally, exploring

Developmental Aspects of Muscles

<

Electron micrograph of a bundle of skeletal muscle fibers wrapped in connective tissue.

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B ecause flexing muscles look like mice scurrying beneath the skin, some scientist long ago dubbed them muscles, from the Latin mus meaning "little mouse." Indeed, we tend to think of the rippling muscles of professional boxers or weight lifters when we hear the word muscle. But muscle is also the dominant tissue in the heart and in the walls of other hollow organs. In all its forms, muscle tissue makes up nearly half the body's mass.

Muscles are distinguished by their ability to transform chemical energy (ATP) into directed mechanical energy. In so doing, they become capable of exerting force.

9.1 There are three types of muscle tissue

Learning Objectives

Compare and contrast the three basic types of muscle tissue. List four important functions of muscle tissue.

Types of Muscle Tissue

Chapter 4 introduced the three types of muscle tissue--skeletal, cardiac, and smooth--and Table 9.3 on pp. 310?311 provides a comparison of the three types. Now we are ready to describe each type in detail, but before we do, let's introduce some terminology.

Skeletal and smooth muscle cells (but not cardiac muscle cells) are elongated, and are called muscle fibers.

Whenever you see the prefixes myo or mys (both are word roots meaning "muscle") or sarco (flesh), the reference is to muscle. For example, the plasma membrane of muscle cells is called the sarcolemma (sarko-lemah), literally, "muscle" (sarco) "husk" (lemma), and muscle cell cytoplasm is called sarcoplasm.

Okay, let's get to it.

Skeletal Muscle

Skeletal muscle tissue is packaged into the skeletal muscles, organs that attach to and cover the bony skeleton. Skeletal muscle fibers are the longest muscle cells and have obvious stripes called striations. Although it is often activated by reflexes, skeletal muscle is called voluntary muscle because it is the only type subject to conscious control.

When you think of skeletal muscle tissue, the key words to keep in mind are skeletal, striated, and voluntary.

Skeletal muscle is responsible for overall body mobility. It can contract rapidly, but it tires easily and must rest after short periods of activity. Nevertheless, it can exert tremendous power. Skeletal muscle is also remarkably adaptable. For example, your forearm muscles can exert a force of a fraction of an ounce to pick up a paper clip--or a force of about 6 pounds to pick up this book!

Cardiac Muscle

Cardiac muscle tissue occurs only in the heart, where it constitutes the bulk of the heart walls. Like skeletal muscle cells, cardiac muscle cells are striated, but cardiac muscle is not voluntary. Indeed, it can and does contract without being stimulated

Chapter 9 Muscles and Muscle Tissue

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by the nervous system. Most of us have no conscious control over how fast our heart beats.

Key words to remember for cardiac muscle are cardiac, striated, and involuntary.

Cardiac muscle usually contracts at a fairly steady rate set by the heart's pacemaker, but neural controls allow the heart to speed up for brief periods, as when you race across the tennis court to make that overhead smash.

Smooth Muscle

Smooth muscle tissue is found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages. Its role is to force fluids and other substances through internal body channels. Like skeletal muscle, smooth muscle consists of elongated cells, but smooth muscle has no striations. Like cardiac muscle, smooth muscle is not subject to voluntary control. Its contractions are slow and sustained.

We can describe smooth muscle tissue as visceral, nonstri- 9 ated, and involuntary.

Characteristics of Muscle Tissue

What enables muscle tissue to perform its duties? Four special characteristics are key.

Excitability, also termed responsiveness, is the ability of a cell to receive and respond to a stimulus by changing its membrane potential. In the case of muscle, the stimulus is usually a chemical--for example, a neurotransmitter released by a nerve cell.

Contractility is the ability to shorten forcibly when adequately stimulated. This ability sets muscle apart from all other tissue types.

Extensibility is the ability to extend or stretch. Muscle cells shorten when contracting, but they can stretch, even beyond their resting length, when relaxed.

Elasticity is the ability of a muscle cell to recoil and resume its resting length after stretching.

Muscle Functions

Muscles perform at least four important functions for the body:

Produce movement. Skeletal muscles are responsible for all locomotion and manipulation. They enable you to respond quickly to jump out of the way of a car, direct your eyes, and smile or frown. Blood courses through your body because of the rhythmically beating cardiac muscle of your heart and the smooth muscle in the walls of your blood vessels, which helps maintain blood pressure. Smooth muscle in organs of the digestive, urinary, and reproductive tracts propels substances (foodstuffs, urine, semen) through the organs and along the tract.

Maintain posture and body position. We are rarely aware of the skeletal muscles that maintain body posture. Yet these muscles function almost continuously, making one tiny adjustment after another to counteract the never-ending downward pull of gravity.

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Stabilize joints. Even as they pull on bones to cause movement, they strengthen and stabilize the joints of the skeleton.

Generate heat. Muscles generate heat as they contract, which plays a role in maintaining normal body temperature.

What else do muscles do? Smooth muscle forms valves to regulate the passage of substances through internal body openings, dilates and constricts the pupils of your eyes, and forms the arrector pili muscles attached to hair follicles.

In this chapter, we first examine the structure and function of skeletal muscle. Then we consider smooth muscle more briefly, largely by comparing it with skeletal muscle. We describe cardiac muscle in detail in Chapter 18, but for easy comparison, Table 9.3 on pp. 310?311 summarizes the characteristics of all three muscle types.

Check Your Understanding

1. When describing muscle, what does "striated" mean? 2. Devon is pondering an exam question that asks, "Which muscle type has elongated cells

and is found in the walls of the urinary bladder?" How should he respond?

For answers, see Answers Appendix.

9.2 A skeletal muscle is made up of muscle fibers, nerves, blood vessels, and connective tissues

Learning Objective

Describe the gross structure of a skeletal muscle.

For easy reference, Table 9.1 on p. 286 summarizes the levels of skeletal muscle organization, gross to microscopic, that we describe in this and the following modules.

Each skeletal muscle is a discrete organ, made up of several kinds of tissues. Skeletal muscle fibers predominate, but blood vessels, nerve fibers, and substantial amounts of connective tissue are also present. We can easily examine a skeletal muscle's shape and its attachments in the body without a microscope.

Nerve and Blood Supply

In general, one nerve, one artery, and one or more veins serve each muscle. These structures all enter or exit near the central part of the muscle and branch profusely through its connective tissue sheaths (described below). Unlike cells of cardiac and smooth muscle tissues, which can contract without nerve stimulation, every skeletal muscle fiber is supplied with a nerve ending that controls its activity.

Skeletal muscle has a rich blood supply. This is understandable because contracting muscle fibers use huge amounts of energy and require almost continuous delivery of oxygen and nutrients via the arteries. Muscle cells also give off large amounts of metabolic wastes that must be removed through veins if contraction is to remain efficient. Muscle capillaries, the smallest of the body's blood vessels, are long and winding and have numerous cross-links, features that accommodate changes in muscle length. They straighten when the muscle stretches and contort when the muscle contracts.

Connective Tissue Sheaths

In an intact muscle, several different connective tissue sheaths wrap individual muscle fibers. Together these sheaths support each cell and reinforce and hold together the muscle, preventing the bulging muscles from bursting during exceptionally strong contractions.

Bone Tendon

Epimysium

Chapter 9 Muscles and Muscle Tissue

281

Epimysium

Perimysium

Endomysium

Muscle fiber in middle of a fascicle

(b) Blood vessel Perimysium wrapping a fascicle Endomysium (between individual muscle fibers)

9

Muscle fiber

Fascicle

(a)

Perimysium

Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium. (b) Photomicrograph of a cross section of part of a skeletal muscle (30?). (For a related image, see A Brief Atlas of the Human Body, Plate 29.)

Practice art labeling >Study Area>Chapter 9

Let's consider these connective tissue sheaths from external to internal (see Figure 9.1 and the top three rows of Table 9.1).

Epimysium. The epimysium (ep-mise-um; "outside the muscle") is an "overcoat" of dense irregular connective tissue that surrounds the whole muscle. Sometimes it blends with the deep fascia that lies between neighboring muscles or the superficial fascia deep to the skin.

Perimysium and fascicles. Within each skeletal muscle, the muscle fibers are grouped into fascicles (fas-klz; "bundles") that resemble bundles of sticks. Surrounding each fascicle is a layer of dense irregular connective tissue called perimysium (per-mise-um; "around the muscle").

Endomysium. The endomysium (endo-mise-um; "within the muscle") is a wispy sheath of connective tissue that surrounds each individual muscle fiber. It consists of fine areolar connective tissue.

As shown in Figure 9.1, all of these connective tissue sheaths are continuous with one another as well as with the tendons that join muscles to bones. When muscle fibers contract, they pull on these sheaths, which transmit the pulling force to the bone to be moved. The sheaths contribute somewhat to the natural elasticity of muscle tissue, and also provide routes for the entry and exit of the blood vessels and nerve fibers that serve the muscle.

Attachments

Recall from Chapter 8 that most skeletal muscles span joints and attach to bones (or other structures) in at least two places. When a muscle contracts, the movable bone, the muscle's insertion, moves toward the immovable or less movable bone, the muscle's origin. In the muscles of the limbs, the origin typically lies proximal to the insertion.

Muscle attachments, whether origin or insertion, may be direct or indirect.

In direct, or fleshy, attachments, the epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage.

In indirect attachments, the muscle's connective tissue wrappings extend beyond the muscle either as a ropelike tendon (Figure 9.1a) or as a sheetlike aponeurosis (aponu-rosis). The tendon or aponeurosis anchors the muscle to the connective tissue covering of a skeletal element (bone or cartilage) or to the fascia of other muscles.

Indirect attachments are much more common because of their durability and small size. Tendons are mostly tough collagen fibers which can withstand the abrasion of rough bony projections that would tear apart the more delicate muscle tissues. Because of their relatively small size, more tendons than

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fleshy muscles can pass over a joint--so tendons also conserve space.

Check Your Understanding

3. How does the term epimysium relate to the role and position of this connective tissue sheath?

For answers, see Answers Appendix.

9.3 Skeletal muscle fibers contain calcium-regulated molecular motors

Learning Objectives

Describe the microscopic structure and functional roles of the myofibrils, sarcoplasmic reticulum, and T tubules of skeletal muscle fibers.

Describe the sliding filament model of muscle contraction.

9 Each skeletal muscle fiber is a long cylindrical cell with multiple oval nuclei just beneath its sarcolemma or plasma membrane (Figure 9.2b). Skeletal muscle fibers are huge cells. Their diameter typically ranges from 10 to 100 m--up to ten times that of an average body cell--and their length is phenomenal, some up to 30 cm long. Their large size and multiple nuclei are not surprising once you learn that hundreds of embryonic cells fuse to produce each fiber. Sarcoplasm, the cytoplasm of a muscle cell, is similar to the cytoplasm of other cells, but it contains unusually large amounts of glycosomes (granules of stored glycogen that provide glucose during muscle cell activity for ATP production) and myoglobin, a red pigment that stores oxygen. Myoglobin is similar to hemoglobin, the pigment that transports oxygen in blood. In addition to the usual organelles, a muscle cell contains three structures that are highly modified: myofibrils, sarcoplasmic reticulum, and T tubules. Let's look at these structures more closely because they play important roles in muscle contraction.

Myofibrils

A single muscle fiber contains hundreds to thousands of rodlike myofibrils that run parallel to its length (Figure 9.2b). The myofibrils, each 1?2 m in diameter, are so densely packed in the fiber that mitochondria and other organelles appear to be squeezed between them. They account for about 80% of cellular volume.

Myofibrils contain the contractile elements of skeletal muscle cells, the sarcomeres, which contain even smaller rodlike structures called myofilaments. Table 9.1 (bottom three rows; p. 286) summarizes these structures.

Striations

Striations, a repeating series of dark and light bands, are evident along the length of each myofibril. In an intact muscle fiber, the

dark A bands and light I bands are nearly perfectly aligned, giving the cell its striated appearance.

As illustrated in Figure 9.2c:

Each dark A band has a lighter region in its midsection called the H zone (H for helle; "bright").

Each H zone is bisected vertically by a dark line called the M line (M for middle) formed by molecules of the protein myomesin.

Each light I band also has a midline interruption, a darker area called the Z disc (or Z line).

Sarcomeres

The region of a myofibril between two successive Z discs is a sarcomere (sarko-mr; "muscle segment"). Averaging 2 m long, a sarcomere is the smallest contractile unit of a muscle fiber--the functional unit of skeletal muscle. It contains an A band flanked by half an I band at each end. Within each myofibril, the sarcomeres align end to end like boxcars in a train.

Myofilaments

If we examine the banding pattern of a myofibril at the molecular level, we see that it arises from orderly arrangement of even smaller structures within the sarcomeres. These smaller structures, the myofilaments or filaments, are the muscle equivalents of the actin- or myosin-containing microfilaments described in Chapter 3. As you will recall, the proteins actin and myosin play a role in motility and shape change in virtually every cell in the body. This property reaches its highest development in the contractile muscle fibers.

The central thick filaments containing myosin (red) extend the entire length of the A band (Figure 9.2c and d). They are connected in the middle of the sarcomere at the M line. The more lateral thin filaments containing actin (blue) extend across the I band and partway into the A band. The Z disc, a coin-shaped sheet composed largely of the protein alphaactinin, anchors the thin filaments. We describe the third type of myofilament, the elastic filament, in the next section. Intermediate (desmin) filaments (not illustrated) extend from the Z disc and connect each myofibril to the next throughout the width of the muscle cell.

Looking at the banding pattern more closely, we see that the H zone of the A band appears less dense because the thin filaments do not extend into this region. The M line in the center of the H zone is slightly darker because of the fine protein strands there that hold adjacent thick filaments together. The myofilaments are connected to the sarcolemma and held in alignment at the Z discs and the M lines.

The cross section of a sarcomere on the far right in Figure 9.2e shows an area where thick and thin filaments overlap. Notice that a hexagonal arrangement of six thin filaments surrounds each thick filament, and three thick filaments enclose each thin filament.

(a) Photomicrograph of portions of two isolated muscle fibers (700?). Notice the obvious striations (alternating dark and light bands).

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Nuclei

Dark A band Light I band

Fiber

(b) Diagram of part of a muscle fiber showing the myofibrils. One myofibril extends from the cut end of the fiber.

Sarcolemma Mitochondrion

(c) Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Each sarcomere extends from one Z disc to the next.

(d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments.

Dark A band

Thin (actin) filament

Light I band Z disc

Nucleus H zone

Thick (myosin) filament

Z disc

I band

A band Sarcomere

M line

Myofibril Z disc

I band

M line Z disc

9

Thin (actin) filament Elastic (titin) filaments Thick (myosin) filament

(e) Cross-sectional view of a sarcomere cut through in different locations.

Myosin filament

Actin filament

I band thin filaments

only

H zone thick filaments

only

M line thick filaments linked by accessory proteins

Outer edge of A band thick and thin

filaments overlap

Figure 9.2 Microscopic anatomy of a skeletal muscle fiber. (For a related image, see A Brief Atlas of the Human Body, Plate 28.)

Practice art labeling >Study Area>Chapter 9

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Z disc

Longitudinal section of filaments within one sarcomere of a myofibril

Z disc

In the center of the sarcomere, the thick filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap.

9 Thick filament

Thin filament

Each thick filament consists of many myosin molecules whose heads protrude at opposite ends of the filament.

Portion of a thick filament

A thin filament consists of two strands of actin subunits twisted into a helix plus two types of regulatory proteins

(troponin and tropomyosin).

Portion of a thin filament

Myosin head

Tropomyosin

Troponin Actin

Actin-binding sites

Heads

Tail

ATP-

binding

site

Flexible hinge region

Myosin molecule

Figure 9.3 Composition of thick and thin filaments.

Actin subunits

Active sites for myosin attachment

Thin filament (actin) Myosin heads

Thick filament (myosin)

Figure 9.4 Myosin heads forming cross bridges that generate muscular contractile force. Part of a sarcomere is seen in a transmission electron micrograph (277,000?).

Chapter 9 Muscles and Muscle Tissue

Molecular Composition of Myofilaments

Muscle contraction depends on the myosin- and actin-containing myofilaments. As noted earlier, thick filaments are composed primarily of the protein myosin. Each myosin molecule consists of two heavy and four light polypeptide chains, and has a rodlike tail attached by a flexible hinge to two globular heads (Figure 9.3). The tail consists of two intertwined helical polypeptide heavy chains.

The globular heads, each associated with two light chains, are the "business end" of myosin. During contraction, they link the thick and thin filaments together, forming cross bridges (Figure 9.4), and swivel around their point of attachment, acting as motors to generate force.

Each thick filament contains about 300 myosin molecules bundled together, with their tails forming the central part of the thick filament and their heads facing outward at the end of each thick filament (Figure 9.3). As a result, the central portion of a thick filament (in the H zone) is smooth, but its ends are studded with a staggered array of myosin heads.

The thin filaments are composed chiefly of the protein actin (blue in Figure 9.3). Actin has kidney-shaped polypeptide subunits, called globular actin or G actin, which bear the active sites to which the myosin heads attach during contraction. In the thin filaments, G actin subunits are polymerized into long actin filaments called filamentous, or F, actin. Two intertwined actin filaments, resembling a twisted double strand of pearls, form the backbone of each thin filament (Figure 9.3).

Thin filaments also contain several regulatory proteins.

Polypeptide strands of tropomyosin (tropo-mio-sin), a rod-shaped protein, spiral about the actin core and help stiffen and stabilize it. Successive tropomyosin molecules are arranged end to end along the actin filaments, and in a relaxed muscle fiber, they block myosin-binding sites on actin so that myosin heads on the thick filaments cannot bind to the thin filaments.

Troponin (tropo-nin), the other major protein in thin filaments, is a globular threepolypeptide complex (Figure 9.3). One of its polypeptides (TnI) is an inhibitory subunit that binds to actin. Another (TnT) binds to tropomyosin and helps position it on actin. The third (TnC) binds calcium ions.

Both troponin and tropomyosin help control the myosin-actin interactions involved in contraction. Several other proteins help form the structure of the myofibril.

The elastic filament we referred to earlier is composed of the giant protein titin (Figure 9.2d). Titin extends from the Z disc to the thick filament, and then runs within the thick filament (forming its core) to attach to the M line. It holds the thick filaments in place, thus maintaining the organization of the A band, and helps the muscle cell spring back into shape after stretching. (The part of the titin that spans the I bands is extensible, unfolding when the muscle stretches and recoiling when the tension is released.) Titin does not resist stretching in the ordinary range of extension, but it stiffens as it uncoils, helping the muscle resist excessive stretching, which might pull the sarcomeres apart.

Another important structural protein is dystrophin, which links the thin filaments to the integral proteins of the sarcolemma (which in turn are anchored to the extracellular matrix).

Other proteins that bind filaments or sarcomeres together and maintain their alignment include nebulin, myomesin, and C proteins.

Sarcoplasmic Reticulum and T Tubules

Skeletal muscle fibers contain two sets of intracellular tubules that help regulate muscle contraction: (1) the sarcoplasmic reticulum and (2) T tubules.

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